Products to be delivered to a customer are generally subjected to a test in regard to operability and durability by a respective manufacturer before delivery. That is particularly important in the case of circuit arrangements with electrical and electronic components, wherein the circuit arrangement can also involve a relatively high degree of complexity and can thus have a large number of different components. Functional testing can therefore be correspondingly complicated and expensive.
In the field of electrical and electronic engineering the individual discrete components as well as integrated circuits are subjected to a detailed test before they are fitted into a device in conjunction with other components or are arranged on a printed circuit board or circuit board (functional assembly). At any event the printed circuit board or the device which is entirely or at least partially finished is to be tested in respect of its function at various stages in manufacture.
That testing operation concerns the general basic function that the circuit arrangement disposed on the printed circuit board (in some cases also a plurality of separate or connected circuit arrangements) must perform. For that purpose actual signals occurring in normal use are applied to the circuit board and, upon evaluation of output signals of the board, it is possible to determine whether the function is performed, in which case it may also be of interest to ascertain the tolerance ranges within which the output signals occur. In such a test, it is based on the usual operating conditions, wherein those operating conditions should also involve the usual prevailing ambient conditions of the respective circuit board in use. The circuit board or products related thereto are delivered only after successful and in part multiple testing.
If it is known that components or circuit boards are used under ambient conditions which lead to an increase loading, such as for example a mechanical loading due to shaking or vibration, or a thermal loading with excessive temperatures, testing is then also necessary to ascertain whether the circuit board or the product connected thereto also performs its function under those extreme ambient conditions and long-term stability can be expected.
For example components and circuit boards arranged in the proximity of a strong heat source are subject to an increase thermal loading which can also give rise to severe fluctuations. Such thermal loadings occur for example in motor vehicles if they are operated in an environment with basically increased temperatures (for example under strong sunlight). Greatly increased temperatures can also briefly occur in an engine compartment of a motor vehicle if for example the internal combustion engine is operated over a prolonged period of time with a high power output in a hot environment and the vehicle has to stop or if a greater amount of power is required at comparatively low engine speeds (and thus with a lower degree of cooling). In that case it is necessary, in connection with a test arrangement for devices to be tested, to implement the corresponding boundary conditions so that checking (testing) can be carried out under the approximately real conditions of a possible increased temperature and also provides information in regard to service life and operability.
A suitable test cycle for testing electrical or electronic components or also a circuit board equipped with such components is determined in dependence on demands and specifications of a customer, in regard to the use of the components or circuit board. In that respect, temperature ranges which are at the edge of the temperature range that occurs in normal operation, in use by the customer, are frequently used for the test. If for example control circuits are considered on a circuit board with corresponding power components and electrolytic capacitors for use in a motor vehicle for the actuation of electric motors for auxiliary modes of operation, then test cycles can be used there, at elevated temperatures of for example 90° C. to 110° C. In that respect, more specifically the circuit boards or also the modular devices are subjected to those changing temperatures in that temperature range and functioning is tested by means of the actuation of corresponding electrical signals and evaluation of the output signals. In that case the output signals which are formed in dependence on the function and the actuation signals in the circuit arrangement to be tested (on the circuit board) are compared to reference or target signals. It is possible to arrive at a conclusion about the operability of the circuit arrangement or circuit board, in dependence on the comparison result.
In the case of a requirement for testing the circuit board at elevated temperatures as are frequently prescribed by customers for the circuit board, the temperature for testing a device, a discrete component or the circuit board is produced in a suitable environment, for example in a test cabinet, in which case the heat produced acts from the exterior on the test piece for example in the form of the circuit board. Temperature detection in the test cabinet and also on the circuit board is required to achieve precise basic data for reliable testing.
If, to simulate real operation, the test piece such as for example the circuit board is actuated with electrical signals, the feed of voltages and currents then produces a corresponding power loss in the device or the circuit board so that this also involves a contribution to the increase in temperature. When determining the temperature of the test piece and its test environment therefore it is necessary to take account of the inherent heat in operation of the test piece (the circuit board).
If the above-specified circuit boards having a circuit arrangement for feeding power to a load, for example to an electric motor of a fan in a motor vehicle, are considered, then besides further requirements there is the need that the circuit arrangement or the circuit board with the corresponding electrical or electronic components must also be operable in a temperature region of 110° C. In that case operable signifies the possibility of feeding a variable power to the connected load like the electric motor in a controlled fashion and thus in connection with a cadence of output signals.
In general service life tests are performed at about 110° C. and concern the properties of the circuit arrangement and the circuit board such as the capability of feeding to a connected load in cyclically controlled fashion, a power which can be controlled or regulated in connection with the cyclic signal cadence. If the circuit arrangement or the circuit board is supplied with corresponding clock signals (for example with pulse width modulated signals or PWM signals) for the controllable or regulatable supply of power to the load and if moreover the situation involves an increased ambient temperature and thus also a circuit board temperature of about 110° C. then interruption of the clock is effected to protect the electrical and electronic components on the circuit board, in which case then to guarantee operation, for example of the load in the form of a fan in the motor vehicle sector, it is actuated at full power.
In that case the power components on the circuit board are completely switched through. The circuit arrangement on the circuit board automatically performs that measure, in which case temperature detection is performed on the circuit board and/or the immediate environment. When the circuit components are completely switched through, in that case there is a lower power loss in respect of the components on the circuit board, irrespective of the maximum power feed to the connected load, so that this entails a lower degree of additional inherent heating as a consequence of the components being completely switched through and the interruption in a clock control. If in operation or also during the testing at the appropriate temperature of about 110° C. that operating condition (ambient conditions) is detected, then for reasons of safety and reliability it is possible to provide for the power components being completely switched on (switched through). That is also justified in view of the high ambient temperature.
That prevents excessive inherent heating of the power components and in particular also the electrolytic capacitors which are subjected to a lower current loading as a consequence of the interruption in the clock control.
If the clock control is interrupted in the limit region of the temperature of about 110° C. and the power components (generally power transistors) are put into the conducting condition and thus switched on, then, on the part of the control circuit formed on the circuit board in question, there is no longer any possibility of controlling or regulating the power of the connected load. Rather, to ensure operation of the load (for example a fan motor) the full power is supplied, so that at least operation of the load is guaranteed, even if the circuit board is in its limit region in regard to its own temperature conditions or the temperature conditions prevailing in the environment.
Thus controllability or regulatability of the power fed to the load is no longer possible by interrupting the clock control, in the temperature limit region of more than 110° C. in conjunction with for example a corresponding test cycle (thermal loading). Rather, after detection of a temperature of more than 110° C. the circuit automatically makes the transition into a simplified mode of operation in accordance with predetermined programming or in conjunction with a control from the exterior, wherein a power which is no longer variable is fed to the load in that simplified mode of operation. In principle complete shut-down is also possible.
Such a behaviour of a circuit arrangement to be tested or a circuit board is shown in FIG. 5. FIG. 5 shows a time-dependent temperature characteristic K (time graph, testing characteristic), as is used in accordance with a known testing procedure. In a comparatively low temperature region of below 110° C., the usual form of actuation of the load connected to the circuit arrangement or circuit board, with clock-controlled signals, is effected between the times t0 and t1, between the times t2 and t3, and from the time t4 (regions A), in which case it is possible to test the operability of the circuit board (circuit arrangement). Thus corresponding clock control of the supplied electrical power (electric signals, output signals) is effected in the regions A in dependence on a power requirement for the load.
In the further time regions which are denoted by B and lie between the times t1 and t2 and between the times t3 and t4, an increased temperature of more than 110° C. occurs, for example a temperature of 120° C. (increased thermal loading). In the regions B, in accordance with the inherent control by the circuit arrangement after detection of the temperature which is occurring or in accordance with control from the exterior, actuation of the connected load with clock-controlled signals no longer occurs as in the regions B the temperature which is increased in any case and which acts on the circuit board from the exterior would entail a danger for the components if inherent heat of the components, for example the electrolytic capacitors and the power transistors, would involve a greater inherent heat (dissipation heat) being added thereto. With an increased temperature range above a predetermined limit temperature therefore the circuit arrangement or the associated circuit board can no longer involve implementation of testing of the controllability or regulatability of the power fed to the load, as a consequence of a necessary interruption in the clock control of the output signals for protecting the components.